Two structurally distinct cellular genes from a rat genomic library can be identified by their close homology to the transforming gene of Harvey murine sarcoma virus. Experiments were designed to investigate the relative ability of these genes to direct the synthesis of the transforming gene product. Under the control of SV40 late regulatory elements, one of these genes (which contains several introns) produces both message and the transforming protein, whereas the other gene, which lacks introns, is transcribed efficiently but is translated only in greatly reduced amounts. This result correlates with the inability of the "intron-less" gene to transform NIH-3T3 cells under the control of a retroviral long terminal repeat and suggests that a mutation which reduces translational efficiency resides either within or very near the coding region of this gene.